U.S. Pat. No. 5,554,327, assigned to the assignee hereof, discloses a method and apparatus for compression molding plastic articles such as plastic closures. A plurality of tools are mounted in a circumferential array on a rotatable turret in a plurality of opposed co-acting pairs. The tools of each pair carry opposed male and female mold sections that together, when closed, form a cavity mold for compression molding the desired articles. The turret is mounted on a machine frame that also carries cams for operative engagement with the tools for moving the tools of each pair toward each other during a portion of motion of the tools around the turret axis for compression molding articles between the tool pairs, and for moving the tool pairs away from each other during another portion of tooling travel for releasing articles molded between the tools. Fluid cylinders are associated with each tool pair to provide a constant limiting molding force for each tool pair. U.S. Pat. No. 5,603,964, also assigned to the assignee hereof, discloses an apparatus for cutting and delivering extruded plastic gobs to the successive molds of the molding apparatus.
Although the methods and apparatuses disclosed in the noted patents address and overcome problems theretofore extant in the art, further improvements remain desirable. For example, it is desirable to hold the tools and mold halves in the closed and compressed position for as long as possible to improve cooling efficiency and part definition and quality during the molding process. However, it is also undesirable to employ the cams on the machine frame for holding the molds closed at normal molding force during most of the operating cycle because this would place undesirable forces on the machine frame and increase cam wear. For this reason, it is conventional practice to reduce mold forces during the curing portion of the cycle, with corresponding reduction in cooling efficiency and part quality. It is also desirable to improve machine efficiency by increasing the tooling density within a machine, and thereby increasing overall productivity per unit of plant floor space. However, employing conventional technology, this would only exacerbate the mold force problem, which in turn would require redesign of the machine frame, cams and bearings.
It is therefore a general object of the present invention to provide a method and apparatus for compression molding plastic articles that permit maintenance of full compression force on the mold elements during the forming and curing of the molded component while reducing wear on the cams that close the mold elements, and while reducing mold clamping reaction forces applied to the machine frame. Another object of the present invention is to provide a method and apparatus of the described character that achieve improved efficiency in terms of tooling density and utilization of plant floor space.